So far, two-stage laser systems comprising an oscillation-stage laser and an amplification-stage laser adapted to amplify laser light emitted out of the oscillation-stage laser have been known so far in the art for the purpose of obtaining high outputs. Two modes, MOPA (Master Oscillator Power Amplifier) and MOPO (Master Oscillator Power Oscillator) are known for double-chamber laser systems. The former is a mode having no resonator in the amplification stage, and the latter is a mode having an unstable resonator in the amplification stage. The MOPA mode and the MOPO mode have merits and demerits over each other.
MOPA
    (a) Low spatial coherence (merit). That is, given the same share quantity (pinhole-to-pinhole spacing) in the beam transverse direction, the visibility of interference fringes is low. Notice that the share quantity and visibility will be explained later.    (b) Low energy stability (demerit). This is because output fluctuations are sensitive to fluctuations of synchronous excitation timing between the chambers.    (c) Output efficiency is lower than that of the MOPO mode; laser (seed) energy from the oscillation-stage laser must be more than that of the MOPO mode (demerit).    (d) Thick spectral line width (demerit). This is because the latter half of a laser pulse from the oscillation-stage laser contains a lot more roundtrips, and so the spectral line width is too narrow to amplify the tail of that latter half.MOPO    (a) High spatial coherence (demerit). That is, given the same share quantity (pinhole-to-pinhole spacing) in the beam transverse direction, the visibility of interference fringes is high.    (b) High energy stability (merit). This is because output fluctuations are insensitive to fluctuations of synchronous excitation timing between the chambers.    (c) Output efficiency is higher than that of the MOPA mode; laser (seed) energy from the oscillation-stage laser can be less than that of the MOPA mode (merit).    (d) Fine spectral line width (merit). This is because the latter half of a laser pulse from the oscillation-stage laser contains a lot more roundtrips, and so the spectral line width is narrow enough to amplify the tail of that latter half.
As described above, the MOPO mode is more favorable than the MOPA mode saving (a) spatial coherence; in other words, it will be more suitable as a light source for semiconductor aligners such as excimer laser or F2 laser, if proper action is taken to reduce the spatial coherence.
However, the MOPO mode has now been found to have problems in conjunction with the use of an unstable resonator as mentioned above. The problems will now be discussed at great length.
In what follows, the “oscillation-stage laser” will be tantamount to the “line narrowing oscillation-stage laser”. A MOPA system, and a MOPO system is basically made up of at least one oscillation-stage laser and one amplification stage or amplification-stage laser. When there is no resonator in the amplification-stage laser, that amplification-stage laser is herein called the amplification stage with no resonance of light. A system having a resonator in the amplification stage is called a MOPO system. When there is a resonator in the amplification stage, the amplification stage functions as an amplification-stage laser with resonance of light. Accordingly, when the amplification stage is compared with the amplification-stage laser, higher efficiency amplification is achievable with the amplification-stage laser than with the amplification stage, given equal excitation energy.
So far, the amplification-stage laser of an excimer laser MOPO system has incorporated an unstable resonator using a concave mirror having a seed light-introduction hole in its center as an input side mirror and a convex mirror as an output side mirror. Such a concave mirror/convex mirror combination of the unstable resonator constitutes a telephoto optical system having a geometrical magnification factor. Having an optical magnification of about 20, the unstable resonator is used for the purpose of efficiently obtaining high-output, high-coherence laser light in the MOPO system. Notice that the unstable resonator has so far been used primarily as a light source for physicochemical researches.
A system having an unstable resonator in an amplification-stage laser has been proposed as a light source for semiconductor aligners, as set forth in patent publication 1. Although this unstable resonator has an optical magnification reduced down to about 10, the inventors' experimentation has suggested that the spatial coherence is not reduced down to any sufficient level.
That is, the object of using the unstable resonator in a conventional MOPO system is to provide efficient amplification of seed light. A concave mirror that forms a part of the unstable resonator is located in the amplification-stage laser to inject the seed light all over the amplification-stage laser gain area, thereby providing efficient amplification of the seed light.
Patent Publication 1    U.S. Pat. No. 2,820,103
Non-Patent Publication 1    “Basics and Applications of Lasers”, translated by Hitoshi Mochizuki and two others, pp. 30-33 (published from Maruzen Co., Ltd. on Jan. 20, 1986)
Non-Patent Publication 2    Sov. J. Quantum Electron. 16(5), May 1986, pp. 707-709
One of the specifications of much importance in a laser system for aligners is in-plane low coherence (spatial coherence) in a laser light profile section. This spatial coherence capability (coherence) is evaluated by comparison of the coherence of a partial beam profile at a given constant distance (share quantity) A in the beam profile. That distance indicated by A is a value determined by element-to-element spacing, etc. in a fly-eye lens used to eliminate brightness variations in an illumination system in a semiconductor aligner such as a stepper. Then, the spatial coherence at two points in the share quantity A is evaluated by visibility defined by the following formula:
                    Visibility        =                              (                                          maximum                ⁢                                                                  ⁢                fringe                ⁢                                                                  ⁢                intensity                ⁢                                                                                          ⁢                                                                                        ⁢                                  I                  max                                            -                              minimum                ⁢                                                                  ⁢                fringe                ⁢                                                                  ⁢                intensity                ⁢                                                                  ⁢                                  I                  min                                                      )                    ÷                      (                                          maximum                ⁢                                                                  ⁢                fringe                ⁢                                                                  ⁢                intensity                ⁢                                                                  ⁢                                  I                  max                                            +                              minimum                ⁢                                                                  ⁢                fringe                ⁢                                                                  ⁢                intensity                ⁢                                                                  ⁢                                  I                  min                                                      )                                              (        1        )            
Notice here that the “fringe intensity” means the intensity of interference fringes upon interference of light from two points. FIG. 71 is indicative in schematic of interference fringes of light from two points at a given share quantity A and their maximum and minimum fringe intensities Imax and Imin, and FIG. 72 is indicative in schematic of interference fringes of light from two points at a given share quantity and their maximum and minimum fringe intensities Imax and Imin, with an added laser portion. More specifically, FIG. 72 is a schematic representation of an optical arrangement for the evaluation of spatial coherence of a laser light source by a Young's interferometer as well as interference fringes of light from two points at a given share quantity (=pinhole-to-pinhole distance) and their maximum and minimum fringe intensities Imax and Imin. Generally, the spatial coherence is determined depending on the size and intensity distribution of a light source, as viewed from the position of a pinhole that is a point of measurement.
FIG. 73 is indicative of the results of measurement of the visibility of a line narrowing laser and the results of measurement in the case of using the line narrowing laser as an oscillation-stage laser to provide amplification in an unstable resonator amplification-stage laser, as obtained in the inventors' experimentation. These results teach that it is required to satisfy the condition that the share quantity from a semiconductor aligner be equal to or greater than A and the visibility be equal to or less than Vt. Usually, the visibility of a single line narrowing laser satisfies this condition. However, when an unstable resonator having a magnification factor of 5 was used in the amplification-stage laser in this experimentation, the share quantity providing a visibility equal to or less than Vt increased up to B. B≈5×A; the share quantity providing the desired visibility equal to or less than Vt increases by the magnification factor of the unstable resonator. In other words, in the arrangement of FIG. 72 wherein the laser portion is added to FIG. 71, the spatial coherence is evaluated while a beam-expanding optical system comprising a combined concave mirror and convex mirror is located between the laser light source and the pinhole. In this case, the size of the light source, as viewed from the pinhole, decreases by the beam magnification factor. Thus, the share quantity providing the same visibility equal to or less than Vt increases by the magnification factor.
In view of the fact that when the unstable resonator is used in the amplification-stage laser, the share quantity increases by a quantity corresponding to the magnification of that unstable resonator, the inventors have made further experiments, using a MOPO system with a stable resonator the optical magnification of which is set at 1 using plane mirrors as both input- and output-side mirrors. As a result, it has been found that the share quantity A equivalent to that obtained with a single oscillation-stage laser, i.e., that of seed light can be achieved with a MOPO system using that resonator (FIG. 73). That is, the inventors have now discovered that as the unstable resonator is used in the amplification-stage laser of the MOPO system, it causes the share quantity to increase by the optical magnification factor of the unstable resonator, and that if a stable resonator is used, this can then be averted. As described later, this finding is one of the rudiments of the invention.
From another angle of view, why the spatial coherence and the share quantity increase with the use of the unstable resonator is now explained.
FIG. 74 is illustrative in (beam profile) section of laser light emitted out of an oscillation-stage laser. Consider now the coherence of laser light P1 and P2′ spaced away by a distance A1 and laser light P1 and P2 spaced away by a distance A2 in the beam profile. As shown in FIG. 75, the laser light P1 and P2′ at a short distance are put in order or substantially equal in terms of wave phase. With increasing distance, however, there is a little wave phase shift even at the same wavelength; laser light P1 and P2 at a relatively long distance are less likely to interfere spatially. In other words, a long pinhole-to-pinhole distance allows for a decrease in the visibility of interference fringes.
In the prior art, the amplification-stage laser resonator was an unstable resonator. As shown in FIG. 78(a), the unstable resonator comprises an input side concave mirror and an output side convex mirror, and is of the type that is capable of geometrically expanding the section of seed light. Accordingly, when both the amplification-stage laser and the oscillation-stage laser are of much the same size in excitation section (discharge section), seed light from the oscillation-stage laser is such that a partial beam portion having a radius 3A is cut out of the general beam section, as shown in FIG. 76. In the section cut out in this way, the closer the laser light P2 is to the laser light P3, the higher the visibility of interference fringe becomes. Although there is a low visibility at a distance A3, the visibility of interference fringe at distance A4 becomes higher with increasing coherence.
As described above, the prior art amplification-stage laser resonator is an expander system; laser light is expanded while high coherence is maintained. As a consequence, the post-amplification laser light P3 diverges to the position of P3′ as shown in FIG. 77, while high coherence is maintained intact. Thus, even when the specifications for coherence are met at a distance A5 in the oscillation-stage laser, high coherence is still maintained even at a distance A6 beyond the distance A5 by the expansion of the beam of seed light after amplification, offering a problem that the specifications for low coherence are not met.
FIG. 78 is illustrative of how seed light (explained with reference to FIGS. 76 and 77) diverges in the amplification-stage laser using an unstable resonator. A laser light section at a position Z1 of the input side concave mirror (FIG. 78(b)) corresponds to FIG. 76, and a laser light section at a position Z2 of the output side convex mirror (FIG. 78(c)) corresponds to FIG. 77. In the prior art, the amplification-stage laser resonator was an unstable resonator. As shown in FIG. 81(a), the unstable resonator comprises an input side cylindrical concave mirror and an output side cylindrical convex mirror. Reference is now made to a resonator of the type that is capable of geometrically expanding the section of seed light in a longitudinal direction. Seed light oscillated out of the oscillation-stage laser is such that, as shown in FIG. 79, the visibility at a distance A3 (share quantity) between laser light P1 and P3 is higher in the section of injection of seed light than in the section of a beam in the amplification-stage laser amplified by the unstable resonator shown in FIG. 80. Given visibility equivalent to that in the oscillation-stage laser, the distance A4 between laser light P1 and P3 becomes long by the magnification factor of the unstable resonator, meaning that the spatial coherence becomes high.
With the prior art two-stage laser system for aligners that relies upon the MOPO mode, the spatial coherence distance becomes long in proportion to the magnification factor at which the beam of seed light is expanded by the unstable resonator, because the unstable resonator is used in the amplification-stage laser. Thus, the prior art two-stage laser is less than satisfactory for light sources for semiconductor aligners.